Analgesia induced by exposure to a novel environment in rats: effect of concurrent and post-training stressful stimulation.

Exposure of rats for 2 min to a new environment (a 50 X 25 X 25-cm box) induced a mild analgesia measured by the tail-flick method. Additional stressful stimuli (0.5 mA, 1.5-s footshocks, light flashes, or tones) presented during the 2-min exposure did not alter the analgesia. However, the postexposure presentation of light flashes or tones, for either 10 s or 2 min, while the animals were alone in a waiting cage, prevented the analgesic response. Similarly, placing the subjects with their conspecifics in the home cage for 2 min after the exposure prevented the analgesic response. The data suggest that the analgesia may represent a physiological correlate of novelty and that the response can be impaired by post-training treatments.